Most conventional coating resins are insoluble in water. Therefore, in general practice they have been dissolved in a suitable organic solvent or dispersed in water with the aid of an emulsifying agent or surfactant in order to provide a coating composition suitable for application to a substrate surface. A serious disadvantage of organic solvent solutions is that they are potentially toxic, flammable, and environmental pollutants.
Water reducible coatings do not present these problems and are, therefore, highly desirable. For this reason various water reducible coating resins, such as the one described in U.S. Pat. No. 4,474,926, have been developed. Water reducible coatings which utilize such resins have been developed for a variety of purposes and have been widely accepted in many applications such as highway striping paint.
There is a need for a coating for metal substrates which provide improved corrosion and rust resistance. Such coatings could be, for example, beneficially utilized in the automotive industry. It would be particularly desirable for such coatings to be of the water reducible type.
For purposes of this patent application, an aqueous coating system is considered to be a colloidal dispersion of a resin in water which can be reduced by the addition of water and which forms a durable coating when applied to a substrate surface. The term aqueous coating system is used herein interchangeably with the term water reducible coating. Other names which are sometimes applied to water reducible coatings are water born, water solubilized, and water dilutable.